There exists a need in pharmaceutical, biotechnological, and other scientific industries to be able to quickly screen, identify, and/or process large numbers or varieties of fluids. As a result, much attention has been focused on developing combinatorial techniques that require efficient, precise, and accurate fluid handling methods. In order to achieve sufficient efficiency and precision, certain disadvantages inherent in conventional fluid handling systems must be overcome. For example, most fluid handling systems presently in use require that contact be established between the fluid to be transferred and an associated solid surface. Such contact typically results in surface wetting that represents a source of unavoidable fluid waste, a notable drawback when the fluid to be transferred is rare and/or expensive. In addition, a number of fluid dispensing systems are constructed using networks of tubing or other fluid transporting conduits. For example, air bubbles can be entrapped or particulates may become lodged in tubing networks, which, in turn, could compromise fluid transport performance and result in diminished or misdirected fluid flow.
Since fluids used in pharmaceutical, biotechnological, and other scientific industries may be rare and/or expensive, techniques capable of handling small volumes of fluids provide readily apparent advantages over those requiring relatively larger volumes. Typically, fluids for use in combinatorial methods are often provided as a collection or library of organic and/or biological compounds. In many instances, well plates are used to store a large number of fluids for screening and/or processing. Well plates are typically single piece in construction and comprise a plurality of identical wells, wherein each well is adapted to contain a small volume of fluid. Such well plates are commercially available in standardized sizes and may contain, for example, 96, 384, 1536, or 3456 wells per well plate.
Pipettes or similar devices are often employed to dispense fluids through an opening into or out of the interior of a well within a well plate. In some instances, complex robotic and/or automated systems may be configured to handle a large number of sample fluids. When a pipetting system is employed during extraction, a minimum loading volume may be required for the system to function properly. Similarly, other fluid dispensing systems may require a certain minimum reservoir volume to function properly. Thus, for any fluid dispensing system, it is important to monitor the reservoir composition and/or volume to determine whether a minimum amount of fluid is provided. Such content monitoring generally serves to indicate the overall performance of a fluid dispensing system, as well as to maintain the integrity of the combinatorial methods.
In addition, when a fluid in a reservoir is exposed to an uncontrolled environment, environmental effects may play a role in detrimentally altering the reservoir composition and/or volume. For example, when a solvent with a low boiling point is used to dissolve or suspend a compound of interest, evaporation of the solvent from the reservoir increases the concentration of compound therein. This, in turn, may cause dissolved compounds to precipitate out of solution, or suspended particles to agglomerate. Thus, for example, when tubing is employed, such precipitation and/or agglomeration may clog the tubing network. Conversely, dimethylsulfoxide (DMSO) is a common organic solvent employed to dissolve or suspend compounds commonly found in drug libraries. DMSO is highly hygroscopic and tends to absorb any ambient water with which it comes into contact. In turn, the absorption of water dilutes the concentration of the compounds as well as alters the ability of the DMSO to suspend the compounds. Furthermore, the absorption of water may promote the decomposition of water-sensitive compounds.
Acoustic technologies may be advantageously employed in fluid handling applications and have been described in a number of patents. For example, U.S. Pat. No. 4,308,547 to Lovelady et al. describes a liquid drop emitter that utilizes acoustic radiation to eject droplets from a body of liquid onto a moving document to result in the formation of characters or barcodes thereon. A nozzleless inkjet printing apparatus is used such that controlled drops of ink are propelled by an acoustical force produced by a curved transducer at or below a free surface of the ink. More recently, acoustic ejection has been employed in contexts other than ink printing applications. For example, U.S. patent application Publication No. 20020037579 to Ellson et al. describes the use of focused acoustic radiation to dispense fluids with sufficient accuracy and precision to prepare biomolecular arrays from a plurality of reservoirs.
Acoustic radiation has also been used to assess the composition and/or volume of one or more fluid reservoirs. For example, U.S. Pat. No. 5,507,178 to Dam describes a sensor for determining the presence of a liquid and for identifying the type of liquid in a container. The ultrasonic sensor determines the presence of the liquid through an ultrasonic liquid presence sensing means, and identifies the type of liquid through a liquid identification means that includes a pair of electrodes and an electrical pulse generating means. This device suffers from the disadvantage that the sensor must be placed in contact with the liquid.
U.S. Pat. No. 5,880,364 to Dam, on the other hand, describes a non-contact ultrasonic system for measuring the volume of liquid in a plurality of containers. An ultrasonic sensor is disposed opposite the top of the containers. A narrow beam of ultrasonic radiation is transmitted from the sensor to the open top of an opposing container to be reflected from the air-liquid interface of the container back to the sensor. By using the round trip transit time of the radiation and the dimensions of the containers being measured, the volume of liquid in the container can be calculated. The device lacks precision because air is a poor conductor of acoustic energy, particularly at the high frequencies required to produce the small wavelengths beneficial to precision measurements. Thus, while this device may provide a rough estimate of the volume of liquid in relatively large containers, it is unsuitable for providing a detailed assessment of the composition and/or volume of small volume reservoirs that are typically used in combinatorial techniques. In particular, this device cannot determine the position of the bottoms of containers since substantially all of the emitted acoustic energy is reflected from the liquid surface and does not penetrate sufficiently to detect the bottom. Small volume reservoirs such as those found in well plates are regular arrays of fluid containers, and the location of the container bottom can vary by a significant fraction of the nominal height of a container due to bow in the plate. In short, detection of only the position of the liquid surface leads to significant errors in height and thus volume estimation in common containers.
More recently, U.S. patent application Ser. No. 10/010,972, “Acoustic Assessment of Fluids in a Plurality of Reservoirs,” inventors Mutz, Ellson, and Foote, filed Dec. 7, 2001, describes the use of an acoustic generator to generate acoustic radiation used to eject fluid from a reservoir or to analyze a property of the fluid content within the reservoir. By analyzing a characteristic of the acoustic radiation transmitted through the fluid, various properties of the fluid within the reservoir may be determined. In addition, such analysis may be carried out to determine the spatial relationship between a free surface of the fluid within the reservoir.
Because dispensation accuracy, precision, and repeatability often depend upon the source from which a fluid is dispensed, there exists a need in the art to control the composition and/or volume of fluid reservoirs. This need is present in various fluid dispensing techniques that involve the handling of small volumes of fluid (e.g., pipettes, capillaries, inkjet printheads, etc.), and is particularly significant when focused acoustic radiation is employed to eject droplets of fluid from a reservoir. Such precise control allows increased robustness, efficiency, and effectiveness of fluid delivery, which is especially valuable for processes such as combinatorial techniques, microfluidic applications, nucleotidic analyses, proteomic studies, and cellular assays.